Cygnus X-1, the black hole
You should read first an excellent
introduction to identification of
Cygnus X-1 as a black hole made by Steven Degennaro.
What is it?
Cygnus X-1 was discovered as an X-ray source 30 years ago
(Bowyer et al. 1965). This binary system, distant of 2.5 kpc, consists of the
O9.7 Iab type supergiant HDE 226868 (Gies & Bolton 1986) and a compact
object orbiting around with a period of 5.6 days. The mass of the unseen
companion, significantly larger then 5 solar masses (Dolan & Tapia 1989,
Dolan 1992) suggests that it is a black hole. Focused wind accretion (Gies &
Bolton 1986) from a primary star being extremely close to filling the Roche lobe
drives the powerful source of the X-ray radiation. Cygnus X-1 is one of the
brightest X-ray sources in the sky.
Emission variability
X-ray states
X-ray emission of Cygnus X-1 exhibits strong variability at
time scales from milliseconds to years. Generally, the X-ray emission falls into
one of the two distinct states, named "low" and "high"(Liang & Nolan 1984).
Cygnus X-1 spends most of its time (>80%) in the low state, where flux in
soft X-ray range (2-20 keV) is lower than in the high state. Ling et al. (1987)
have divided the hard X-ray luminosity (45- 140 keV) into three states: gamma1,
gamma2, and gamma3. Recent observations (Phlips et al. 1995) suggest that
variability does not seem to be between discrete states but rather among
continuous range of possible flux values.
Intensity dips
Another temporal feature of Cygnus X-1 are intensity dips
(Li & Clark 1974) that preferentially occur near the time of superior
conjunction. The duration of the dip may vary from minutes to hours. The flux
during the dip decreases in soft X-ray range (E<10 keV), and absorption is
generally complex. The partial covering model is consistent with the
observational data (Kitamoto et al. 1984). The absorber seems to form a dense
blobs of matter, intervening the X-ray source (Pravdo et al. 1980).
The spectrum
X-rays
The intrinsic X-ray spectrum of Cygnus X-1 can be characterized
as a power law of the photon index Gamma ~1.5-1.9 (Liang & Nolan 1984). This
kind of the spectrum can be explained by Comptonization of soft photons,
presumably coming from the optically thick accretion disk. On top of this
continuum one can find an absorption edge at around 7 keV, and a broad feature
above 10 keV, called high energy excess (Done et al. 1992). Recent wide-band
observations show that the excess has a form of the hump extending from 10 to
200 keV, with the peak at ~30 keV (Gierlinski et al. 1995). Such a hump is
considered due to reprocessing and Compton reflection of X-ray photons by an
accretion disk (Lightman, White 1988, Magdziarz, Zdziarski 1995).
Iron line
Barr, White, and Page (1985) reported a wide (equivalent width
~120 eV) emission iron K-alpha line at ~6.2 keV. Fabian et al. (1989) attributed
this line to the fluorescence of the inner part of the accretion disk, at a few
Schwartzschild radii. Such an emission should exhibit a characteristic,
double-wing profile. More recent work (Ebisawa et al. 1995) show that the
observed feature is consistent rather with the narrow gaussian line (1 sigma
< 0.2 keV), suggesting that it can come from the outer part of the disk. The
weakness of the line (equivalent width ~20 eV), inconsistent with theoretical
predictions (George, Fabian 1991), is not understand well.
Gamma-rays
At higher energies (E > 100 keV) observed flux falls down
exponentially. In most cases gamma-rays from Cygnus X-1 can be approximated by a
power law with exponential cutoff. Haardt et al. (1993) showed, using Monte
Carlo methods, that inverse Compton continuum from the optically thin (tau=0.3)
and hot (kT=150 keV) plasma is consistent with SIGMA and OSSE observations.
Something around 1 MeV
Several authors have reported a hardening of the
spectrum in the region around 1 MeV (Ling et al. 1987, Bassani et al. 1989),
though many other observations have found no evidence for such emission (e.g.
McConnel et al. 1994). It suggests that this spectral feature must be transient.
Liang and Dermer (1988) proposed explanation of the 1 MeV bump in context of the
hot (kT ~ 400 keV) electron-positron plasma in the inner region of the accretion
disk.
Cygnus X-1 and Active Galactic Nuclei
Many of the spectral features
mentioned above have been also found in the spectra of Active Galactic Nuclei
(see e.g. Mushotzky, Done, Pounds 1993, Zdziarski 1995). Both in the case of the
Cygnus X-1 and an AGN an X-ray emission seems to origin from the accretion onto
black hole. We observe a power-law intrinsic spectrum, a disk reflection feature
and an iron line. We are convinced that research on this "miniature AGN" can
turn out a significant contribution to the understanding of AGNs.
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